Propylene glycol is an industrially important commodity chemical which until recently has been made only downstream of conventional fossil fuel operations. As a result of significant research efforts, however, biobased propylene glycol is now commercially available.
Such biobased, renewably sourced materials can be differentiated from their petroleum-derived counterparts, for example, by their carbon isotope ratios using ASTM International Radioisotope Standard Method D 6866, the disclosure of which is incorporated by reference in its entirety. Method D 6866 is based upon the fact that isotopic ratios of the isotopes of carbon within any given material, such as the 13C/12C carbon isotopic ratio or the 14C/12C carbon isotopic ratio, can be determined using certain established analytical methods, such as isotope ratio mass spectrometry, with a high degree of precision.
ASTM Method D 6866, similar to radiocarbon dating, compares how much of a decaying carbon isotope remains in a sample to how much would be in the same sample if it were made of entirely recently grown materials. The percentage is called the biobased content of the product. Samples are combusted in a quartz sample tube and the gaseous combustion products are transferred to a borosilicate break seal tube. In one method, liquid scintillation is used to count the relative amounts of carbon isotopes in the carbon dioxide in the gaseous combustion products. In a second method, 13C/12C and 14C/12C isotope ratios are counted (14C) and measured (13C/12C) using accelerator mass spectrometry. Zero percent 14C indicates the entire lack of 14C atoms in a material, thus indicating a fossil (for example, petroleum based) carbon source. One hundred percent 14C, after correction for the post-1950 bomb injection of 14C into the atmosphere, indicates a modern carbon source. ASTM D 6866 effectively distinguishes between biobased materials and petroleum derived materials in part because isotopic fractionation due to physiological processes, such as, for example, carbon dioxide transport within plants during photosynthesis, leads to specific isotopic ratios in natural or biobased compounds. By contrast, the 13C/12C carbon isotopic ratio of petroleum and petroleum derived products is different from the isotopic ratios in natural or bioderived compounds due to different chemical processes and isotopic fractionation during the generation of petroleum. In addition, radioactive decay of the unstable 14C carbon radioisotope leads to different isotope ratios in biobased products compared to petroleum products. As used herein, “biologically derived”, “bioderived”, and “biobased” may be used interchangeably to refer to materials whose carbon content is shown by ASTM D 6866, in whole or in significant part (for example, at least about 20 percent or more), to be derived from or based upon biological products or renewable agricultural materials (including but not limited to plant, animal and marine materials) or forestry materials.
Processes have been developed through the aforementioned research efforts to make a biobased propylene glycol both from glycerol as produced as a byproduct in the manufacture of biodiesel, as well as to make biobased propylene glycol from sugars. The availability of a reasonably affordable glycerol feedstock however depends on a strong global demand for biodiesel, and biodiesel process economics and demand have been affected by changing regulatory environments and governmental programs and initiatives, so that processes to make biobased propylene glycol from sugars have been the subject of significant research work.
The processes that have been proposed in the art as a consequence of this significant research work have almost all involved a plurality of steps. In a first step, for example, in a typical process for converting dextrose or fructose to a biobased propylene glycol, conventionally the six carbon sugar is first hydrogenated to a six carbon sugar alcohol such as sorbitol, and then the sorbitol undergoes a separate hydrogenolysis step (typically in a second reactor) under a second set of conditions to yield lower molecular weight polyols inclusive of propylene glycol.
U.S. Pat. No. 7,038,094 to Werpy et al., for example, describes the conversion of six carbon chain sugar alcohols such as sorbitol to polyols inclusive of propylene glycol, using a multimetallic rhenium-containing catalyst. Other references of a similar nature but using different catalyst systems include U.S. Pat. Nos. 5,206,927, 4,476,331, and European Patent Applications EP-A-0523 014 and EP-A-0 415 202, though many other examples could be cited without difficulty.
One notable such example is U.S. Pat. No. 4,430,253 to Dubeck et al. (“Dubeck”), which describes a process for the production of a lower polyhydric alcohol or a mixture thereof by the hydrogenation and hydrogenolysis of a carbohydrate in two stages. The first stage hydrogenation produces higher polyhydric alcohols such as sorbitol, wherein the catalyst may be a “well-known hydrogenation catalyst”. Examples given include ruthenium, nickel, cobalt and copper catalysts, with ruthenium on carbon catalysts specifically named along with copper on alumina and copper chromite catalysts. In the second stage, the higher polyhydric alcohols undergo hydrogenolysis to desirable lower polyhydric alcohols; sorbitol, for example, is converted to ethylene glycol and propylene glycol. A preferred catalyst for the second stage conversion of sorbitol to ethylene glycol and propylene glycol is a sulfide-modified ruthenium catalyst. In one embodiment that is described, a ruthenium on carbon catalyst used for the first stage hydrogenation is sulfide-modified by the addition of a sulfide-containing solution, typically following the introduction also of a base promoter such as calcium oxide. In another embodiment, the sulfide modified ruthenium catalyst can be “completely prepared prior to the addition of the polyhydric alcohol solution,” col. 7, lines 53-55.
A couple of examples of single-step processes may, however, be found in the literature. In Zhou et al., “Selective Production of 1,2-Propylene Glycol from Jerusalem Artichoke Tuber Using Ni—W2C/AC Catalysts”, ChemSusChem 2012, vol. 5, pp. 932-938 (2012), Zhou et al. referenced as background a 2006 study by Fukuoka et al. wherein a single vessel process was described for accomplishing the hydrolysis of cellulose to provide hexose sugars and the hydrogenation of those sugars to the corresponding hexitols, and then proceeded to describe their discovery of a process for accomplishing the hydrolysis of cellulose, the breaking (cracking) of C—C bonds of the hexoses, and hydrogenation of these hexose hydrogenolysis products to provide ethylene glycol and propylene glycol. Where the predominant hexose obtained from the biomass was glucose (or dextrose), ethylene glycol was found to be the main product, whereas the processing of the inulin-based Jerusalem artichoke biomass was said to provide propylene glycol as the main product. Under optimized conditions, the maximum yield of propylene glycol was reported to be as high as 38.5%, with a combined yield of EG and PG of 52.6%. The catalyst used by Zhou et al. was described as a nickel promoted W2C on activated carbon (AC) catalyst, with the best results reported with a 4% Ni-20% W2C/AC catalyst at 245 degrees Celsius, 6 MPa hydrogen and a reaction time of 80 minutes.
The nickel was found to be necessary to catalyze the hydrogenation of acetol; however, the nickel was also found to promote sintering so that a balancing of positive and negative effects was indicated. Other transition metals such as Pt and Ru were found in substitution of nickel to produce a significant amount of hexitols but no acetol and significantly lower propylene glycol yields. Activated nickel supported Ni nanoparticle catalysts were also found to be effective for both the C—C cracking of sugars and for the hydrogenation of acetol to propylene glycol, though nickel-promoted W2C catalysts were described as exhibiting a synergistic effect as compared to nickel or W2C catalysts alone.
The capacity to make biobased propylene glycol from a high fructose feedstock is of considerable commercial interest since high fructose syrups are commercially made and used as sweeteners throughout the world under various customary names—being commonly referenced as high fructose corn syrup (HFCS) in the United States, glucose-fructose in Canada, isoglucose, glucose-fructose syrup or fructose-glucose syrup in Europe and as high fructose maize syrup in some countries. Nevertheless, in recent years such syrups have been identified by some as contributing to a tendency toward obesity as well as blamed for a number of other adverse health effects, so that a simple, one-step process for converting a high fructose feedstock to a biobased propylene glycol would be extremely desirable—and especially if the process were well-adapted to make efficient use of the high fructose streams or products that are currently available, so that in the event of a decreased sweetener demand, an alternative beneficial use can be made of these with a minimum of additional effort and expense.
HFCS in this regard consists of 24% water and the rest sugars. The most widely used varieties of HFCS are: HFCS 55 (mostly used in soft drinks), approximately 55% fructose and 42% glucose; and HFCS 42 (used in beverages, processed foods, cereals, and baked goods), approximately 42% fructose and 53% glucose. HFCS-90, approximately 90% fructose and 10% glucose, is also commercially used as a product in small quantities for specialty applications, but primarily is found in current production as a blendstock with HFCS 42 to make HFCS 55. Consequently, a “high fructose feedstock” for purposes of the present invention will be understood as including mixtures of fructose with one or more additional sugars wherein the fructose is at least about 42 percent by weight of the sugars as a whole.